20 April. Station 2-1/2 miles ahead of convoy. At daybreak took position on flank. At 7:40 a.m., joined by 4 British destroyers. At 9:00 a.m. five ships bound for ENGLISH CHANNEL left convoy escorted by British destroyers. At 2:18 sighted submarine on surface ahead. Went to general quarters.ZINNIA opened fire. Submarine fired recognition signal, proved to be an American boat. Crossed track of three other convoys during evening watch...